1. Field of the Invention
The present invention relates to a background-calibrated comparator (BCC) and a background-calibrated flash analog-to-digital converter (ADC), which are used on design of mixed-signal integrated circuit, in particular on the high-speed analog-to-digital converter circuit.
2. Discussion of Related Art
U.S. Pat. No. 6,459,394B1 entitled “Multi-bank flash ADC array with uninterrupted operation during offset calibration and auto-zero” has disclosed a flash ADC array in which the comparators are separated into two groups. When one group of comparators is processing calibration, the other group of comparators can maintain the operation of the ADC. However, it will degrade the resolution of the ADC for half. Also, this method requires calibration in approximately every 400 ms, which means the entire resolution of the ADC will degrade periodically and the method thus can only be used in the intermittent system of reading a disk.
U.S. Pat. No. 6,084,538 entitled “Offset calibration of a flash ADC array” uses conventional means of capacitor storage to calibrate and expiate the offset voltage of a single comparator. To achieve the object of background-calibration, the calibration circuit takes turns to selectively calibrate each single comparator. The comparators need to be split from the signal path during calibration and be rejoined after the calibration is done. To the entire ADC, there exists a “bubble” in the output thermometer codes because there is always a comparator in calibration. The encoder in the downstream part thus requires special design to suppress the error caused by the bubble. This method causes great complexity and large amount of wires in the circuit, whereby the operation speed is restrained.
“Offset calibrating comparator array for 1.2-V, 6-bit, 4-Gsample/s flash ADCs using 0.13-μm CMOS technology” by H. Okada, Y. Hashimoto, K. Sakata, T. Tsukada, and K. Ishibashi, ESSCIRC 2003 Proceedings, September 2003, pp. 711–714 has disclosed a pre-calibration method, wherein the calibration circuit generates a triangle wave as an input signal before the circuit operation initiates. The value of the offset voltage of each comparator is determined by the conversion property of the ADC, whereby the reference voltage of each comparator is adjusted to expiate its offset. Since pre-calibration is adopted in this method, the calibration cannot be carried once the circuit operation initiates.
“A 2.5-V 12-b 54-Msample/s 0.25-mm CMOS ADC in 1-mm2 with mixed-signal chopping and calibration,” by H. van der Ploeg, G. Hoogzaad, H. A. H. Termeer, M. Vertregt, and R. L. J. Roovers, IEEE J. Solid-State Circuits, vol. 36, pp. 1859–1867, December 2001 has disclosed a two-step ADC with background calibration. In the paper, a random chopping method similar to that of the present invention is adopted to calibrate the residue amplifier. However, the object to be calibrated in this paper is an amplifier, the offset information is therefore easy to be obtained in the process of random chopping, unlike in the present invention, where the offset is hidden among big quantization noises and is difficult to be obtained since the object to be treated in the present invention is a comparator.
Additionally, the calibration loop in this paper employs the manner of periodical adjustment, which periodically detects if the accumulated output of the amplifier exceeds a predetermined threshold and adjusts the offset accordingly. However, compared with the present invention, the calibration method disclosed in this paper is not efficient enough because the offset cannot be instantly adjusted due to the periodical adjustment.